This invention relates to an electronic memory unit intended primarily for use with an XY plotter to record transient electrical signals.
The measurement and recording of transients is an important requirement in many scientific and engineering fields. Frequently it is necessary to record one transient time-varying signal as a function of another related time-varying signal, as may be required for example in carrying out tests on structural materials or when monitoring chemical reactions. A problem commonly associated with test signals of a transient nature is that the signals may exhibit relatively high rates of change during one important brief portion of the test period, and relatively low rates of change at other times,. If, under these circumstances, an XY plotter is used to plot the curve, the mechanical limitations of the plotter, specifically the speed at which it can respond to input signals, often prevent the accurate recording of that brief portion of the curve when the signals are changing rapidly. One method which has been used to overcome this disadvantage is first of all to record the test electronically by storing the two variables as functions of time with an instrument employing a digital memory. This instrument is set to record the signals with a fixed sampling rate throughout the test, the sampling process being initiated, or triggered, when the initial edge of one of the signals is detected. When the test has been completed, the instrument may then be set to "play back" the signals at a sufficiently slow rate to enable the XY plotter to follow the rapidly changing parts of the signals. The above method is however wasteful of the memory capacity. The sampling rate must be chosen to be sufficiently fast to reproduce the portion of rapid change accurately, but at other times this rate is unnecessarily high since the signals are varying much less rapidly. In most cases, due to a limitation on memory capacity, a compromise has to be made in choosing the sampling rate-between a rate which is fast enough for satisfactory resolution and one which is slow enought to enable the storing of the whole test period without running out of memory capacity. Furthermore, the method is relatively complicated to carry out in that the operator must set the sampling rate, the duration of the sampling process and the triggering system; also when the test is completed the instrument has to be switched over to the playback mode.